I. Field of the Invention
The present invention relates to a circuit breaker, and more particularly to a circuit breaker of a current-limiting type, which can open quickly by using the magnetic repulsive forces generated due to the flow of currents in opposite directions through a fixed contactor and a movable contactor arranged in parallel with the fixed contactor.
II. Description of the Related Art
A conventional circuit breaker is shown in FIG. 6, which illustrates a longitudinal sectional view of an important part of the circuit breaker, and in FIG. 7, which illustrates an enlarged sectional view of the important part.
As shown in the drawings, a fixed contactor 2 is formed in a bent or U-shape and has a fixed contact point 1 secured on an end portion of an upper leg of the U-shaped contactor 2. A movable contactor 6 has a movable contact point 3, adapted with and separated from the fixed contact point 1, which is secured on an end portion of the movable contactor 6. The movable contactor 6 has a portion which is placed in parallel with the fixed contactor 2 when the circuit breaker is closed. A first supporting shaft 4 passes through an intermediate portion of the movable contactor 6 and is secured to a holder 10. A guide pin 5 passes through another end portion of the movable contactor 6 and is fixed to the end portion.
The movable contactor 6 is also oscillatably mounted on a holder 10 through the first supporting shaft 4 and a second supporting shaft 8, which is inserted through a movable contactor guide 7 for supporting one end portion of the movable contactor 6. The holder 10 is made of synthetic plastic material and has a rotary shaft 9 integrally formed on the holder 10. The movable contactor guide 7 is formed in a shape of a U and has a pair of longish openings 7a provided in both legs of the movable contactor guide 7 so as to slidably guide the guide pin 5.
A contact spring 11 is resiliently installed between a bottom surface of the movable contactor guide 7 and the holder 10 in order to exert a force perpendicular to the longish openings 7a as shown by an arrow in FIG. 7. This force gives a contact pressure on the movable contactor 6 by a counterclockwise moment around the first supporting shaft 4 when the movable contactor 6 is kept in contact with the fixed contactor 2 and when these contactors are closed. When the circuit breaker is shortcircuited, large opposing currents flowing through the parallel portions of the contactors 2 and 6 generate a magnetic repulsive force F (See FIG. 1), which clockwise rotates the movable contactor 6. Consequently, the guide pin 5 is shifted to the position as shown by the broken lines in FIG. 7. As a result, the point of application is shifted to this position and the force shown by an arrow with broken line produces clockwise moment around the supporting shaft 4 through the guide pin 5. This clockwise moment accelerates a quick opening of the circuit breaker.
In the conventional circuit breaker mentioned above, it is necessary to shift the point of application and reverse the direction of the moment while the fixed contactor 2 and the movable contactor 6 are within the opening distance of the circuit breaker in which the magnetic repulsive force generated between the two contactors is greater than the contact pressure. As a result, the rotational angle necessary to reverse the direction of the moment or the length for shifting the point of application is restricted when the circuit breaker opens in a current-limiting manner. This makes the length L1 of a perpendicular dropped from the center of the first supporting shaft 4 to the line of actions short and necessitating a strong force from the contact spring 11. As a consequence, a conventional circuit breaker requires the generated magnetic repulsive force to be larger than the strong contact pressure of the contact spring and to be continued until the guide pin 5 is shifted to the position where the direction of the moment is reversed, thereby making the current value for starting a current-limiting action large and making it difficult to manufacture a small-sized circuit breaker with a good current-limiting characteristic.